A mouse monoclonal cell line (20-1.5) was produced by the cell fusion method and the antibody secreted by this line defined the Ly-9.2 specificity--the reciprocal specificity to that previously identified as the Lgp 100 or the T100 molecule. Although most concentrated on lymph-node cells, the antigen is also found on thymocytes, spleen and bone-marrow cells as well as liver and brain tissue. The monoclonal antibody precipitates a 100,000 molecular moiety from thymocytes. The antigenic specificities appear to be highly immunogeneic and antibodies to these specificities contaminate many antisera. These sera are noncytotoxic as is the case with the monoclonal antibody even though it is of the IgG2a subclass. As with T100 or Lgp 100, the Ly-9 locus appears to be linked to the H-25 locus.